Comparative morphology, histology and growth of the dental plates of the devonian dipnoan Chirodipterus

Abstract

The dental plates of the Devonian lungfish Chirodipterus australis Miles (Osteichthyes; Dipnoi) are shown to have achieved their characteristic morphology by a growth process different from that assumed for the plates of genera such as Dipterus . Each plate was thickened by the addition of layers of bone that also extended the plate labially, thus providing the base on which and into which dentine grew. Distinctive features of the dentition are: ( a ) labial increase of the dentine mass by the addition of blister-like denticles of simple enamel-covered dentine, which is initially ingrown by pleromic dentine and subsequently resorbed and replaced by petrodentine; ( b ) increase in the midline by a similar process that results in the addition of one (or possibly two) new ridges; ( c ) resorption of the posterior edge of the pterygoid plates and the posterior and posteromedial edges of the prearticular plates, with subsequent development over the resorbed surfaces of several generations of simple regenerative dentine; ( d ) resorption and redeposition of pleromic dentine and bone in a triangular region posteromedially on the pterygoid plates; ( e ) the formation of tuberosities that simulate teeth at a short distance in from the labial edge, by four processes: formation of an undulating plate margin, differential growth of petrodentine (hard compact dentine) within the pulp cavity, differential wear of the petrodentine and the adjacent bone plus pleromic dentine, and slightly greater growth of the petrodentine towards the occlusal surface relative to the adjacent bone and dentine; ( f ) expansion of the large flat surfaces of the plates by gradual replacement of the bone and dentine at the proximal ends of the furrows and also by the development of linkages of petrodentine across the furrows; ( g ) development of isolated tuberosities on the flat posterolateral parts of the plates. The petrodentine of the ridges, tuberosities and plateaus of the plates is indistinguishable structurally and in its mode of growth from the petrodentine in extant species of dipnoans. Plates similar to those of C. australis have been observed in Stomiahykus , Archaeonectes , Conchodus , Palaedaphus and Sunwapta , as well as several species usually referred to as Dipterus. Sunwapta may be congeneric with C. australis We propose that the term ‘dental plate’ be used as a general term to cover the crushing plates of dipnoans; that the term ‘ tooth plate’ be restricted to those types that grew by the addition of true teeth to the labial margins of the plates, to form radiate rows; and that the term ‘dentine plate’ be used for those that added dentine to their margins without the formation of teeth. The oldest known genera with tooth plates and dentine plates are Speonesydrion and Dipnorhynchus respectively.